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United States Patent |
5,680,459
|
Hook
,   et al.
|
October 21, 1997
|
Passive transponder
Abstract
The present invention relates to an electronic identification system
comprising one or more transponders which store identification and other
information in memory, and readers, which capture information from the
transponders and write new information thereto, and in which
communications between transponders and readers are accomplished using
radio frequency signals. The present invention has attributes which make
it particularly useful in automatic fare collection systems, where
transponders, typically in the form of thin cards and used as pre-paid
tickets, are carried by fare paying passengers to replace printed tickets
and eliminate exchange of cash. Specifically the system achieves
simultaneous identification of numerous transponders by one reader,
permits a reader to selectively program any of many transponders under its
control, exhibits fast transaction speed to maximize passenger throughput,
and achieves exceptional data storage integrity.
Inventors:
|
Hook; Christopher (Reading, GB2);
Juson; Keith (Malmesburg, GB2);
Hall; Chris (London, GB2);
Ferguson; Donald Harold (Maple, CA);
Paun; Dimitrie Octavian (Mississauga, CA);
Oprea; Alexandru (Willowdale, CA)
|
Assignee:
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Kasten Chase Applied Research Limited (Mississauga, CA)
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Appl. No.:
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430825 |
Filed:
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April 28, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
705/65; 705/13 |
Intern'l Class: |
H04K 001/00 |
Field of Search: |
380/23,49
|
References Cited
U.S. Patent Documents
4598275 | Jul., 1986 | Ross et al. | 340/573.
|
Foreign Patent Documents |
0 420 295 A1 | Mar., 1986 | EP.
| |
0 537 378 A1 | Oct., 1991 | EP.
| |
0 578 457 A2 | Jul., 1992 | EP.
| |
0 502 518 A2 | Sep., 1992 | EP.
| |
2 607 946 A1 | Jun., 1988 | FR.
| |
2163324 | Feb., 1986 | GB.
| |
WO88/04453 | Jun., 1988 | WO.
| |
WP90/09707 | Aug., 1990 | WO.
| |
WO91/17515 | Nov., 1991 | WO.
| |
Primary Examiner: Cain; David C.
Attorney, Agent or Firm: Fay, Sharpe, Beall, Fagan, Minnich & McKee
Claims
We claim:
1. In a radio frequency transponder system including an input for receiving
a radio frequency signal modulated with data, said data including an error
check field, a circuit for demodulating said signal so as to recover said
data in serial format, and data memory for storing said data, the
improvement comprising a first circuit for receiving said data in serial
format and converting said data to parallel format, a second circuit for
assessing any errors in said data with reference to said error check field
and in the event said data is free of errors then storing said data in
parallel format in said memory as a succession of pages each comprising
one or more bytes of said data.
2. The improvement of claim 1, wherein said first circuit comprises a
scratchpad memory for bit-serially receiving and accumulating said data to
form a page of said data for parallel storage in said data memory.
3. The improvement of claim 1, wherein said second circuit comprises
control and error check logic for performing a cyclic redundancy check on
said error check field for assessing any said errors in said data, and an
address decoder and latch for storing said data in said data memory.
4. A method of transferring data from a reader to at least one transponder
in a communication system, comprising the steps of:
a) transmitting a packet of said data from said reader to said transponder,
wherein said data includes an error check field;
b) assessing within said transponder whether said data contains any errors
with reference to said error check field;
c) in the event said data is free of errors then storing said data in said
transponder and transmitting an acknowledgement message from said
transponder to said reader for reporting completion of said storing of
said data; and
d) in the event said data contains an error then transmitting a
predetermined message from said transponder to said reader for indicating
that said data has been corrupted, thereby initiating re-transmission of
said packet of data from said reader to said transponder.
5. In a radio frequency transponder for receiving data from a reader, said
data including an address for identifying said transponder from among a
plurality of transponders, the improvement comprising a fixed code area in
said transponder for storing a unique identification number for said
transponder, and logic circuitry for comparing said address included in
said data received from said reader with said identification number so as
to thereby ascertain whether said data is intended for said transponder
from among said of transponders.
6. The improvement of claim 5, wherein said unique identification number is
stored in said fixed code area of said transponder via laser fusing.
7. The improvement of claim 6, wherein said address is encrypted and
wherein said transponder further includes security logic circuitry for
decrypting said address and encrypting said identification number.
8. In a passive transponder system for receiving a radio frequency signal
modulated with programming instruction data from a reader, deriving
operating power from said radio frequency signal, demodulating said
programming instruction data from said radio frequency signal and in
response effecting internal re-programming, the improvement comprising:
a) an energy level detector for detecting energy available in said radio
frequency signal; and
b) control connected to said energy level detector for determining whether
sufficient energy is available in said radio frequency signal to effect
said re-programming in accordance with said programming instruction data,
and in the event sufficient energy is available then effecting said
re-programming, and in the event that insufficient energy is available
then transmitting a message to said reader for notifying the reader that
re-programming cannot be effected.
9. In a passive transponder system having an antenna for receiving a radio
frequency signal modulated with data a reader, circuitry for deriving
operating power from said radio frequency signal, and further circuitry
for demodulating said data from said radio frequency signal, the
improvement comprising:
a) an energy level detector for detecting energy available in said radio
frequency signal; and
b) an automatic gain circuit connected to said energy level detector and
said antenna for varying load resistance to said antenna by increasing
said resistance when said energy available is less than a predetermined
amount, and decreasing said resistance when said energy available is
greater than said predetermined amount, thereby dynamically varying loaded
quality factor (Q) of said transponder relative to distance between said
reader and said transponder so as to provide a wide dynamic operating
range of said transponder.
10. A communication system comprising:
a) a reader for generating a radio frequency excitation field;
b) a plurality of radio frequency transponders, each of said transponders
being capable of transmitting a message to said reader immediately upon
detecting said excitation field; and
c) circuitry within each of said transponders for detecting transmission of
messages to said reader by other ones of said transponders and in response
delaying transmission of said message to said reader by a predetermined
time period, thereby avoiding interference between said plurality of
transponders.
Description
FIELD OF THE INVENTION
The present invention relates generally to radio frequency transponder
systems, in which there are a multitude of transponders and readers. More
particularly, the invention relates to such systems wherein each
transponder essentially comprises circuitry for radio communications and a
control circuit in the form of a silicon integrated circuit (IC), in which
identification information and variable data is stored, for use in
automatic fare collection (AFC) systems, and wherein each fare-paying
passenger is issued with such a transponder, typically in the form of a
thin plastic card, similar to a familiar credit or debit card.
BACKGROUND OF THE INVENTION
There are many transponder systems commonly available which use a variety
of techniques to achieve identification. However, such prier art
transponder systems suffer from a number of deficiencies which make them
unsuitable for use in AFC systems.
As far as is known by the inventors named in this application, there is no
prior art passive (field-powered) system which achieves simultaneous
identification of numerous transponders by one reader, making it
impossible in the prior art to satisfactorily handle a situation where
more than one transponder is presented to a reader at one time, either
accidentally or deliberately. For example, PCT/AU90/00043 (Turner, et al)
describes an identification system which employs transponders that maybe
field-powered (passive) or self-powered (active), in which a plurality of
transponders may clearly be present near a reader, yet the system can only
achieve error-free identification when just one transponder is under its
influence and therefore this system is deficient when applied to AFC
systems and other applications where many transponders may be under the
simultaneous influence of one reader.
Active (self-powered) transponders systems are also known in the art, such
as co-pending patent application POT/CA91/00147 filed in the name of the
same assignee as the present application, which describes an innovative
mechanism by which superior simultaneous identification performance is
achieved. However, active transponders are unsuitable in AFC systems
because of a number of factors, such as: limited operating life due to use
of an exhaustible power source in the form of an encapsulated cell;
transponder thickness which is limited by the thickness of the power cell;
and added cost of the cell.
No prior art system is known to the inventors named herein which permits
the reader to selectively write information to any of many transponders
under its control, an essential corollary after achieving simultaneous
identification. For example, PCT/US87/00466 (Froelich, et al) describes a
transponder which employs photoelectric transducers for data
communications. There is no consideration given in this reference to the
need to selectively programme one of many transponders since this prior
art system requires line of sight between the reader and each transponder.
Prior art passive (field-powered) transponder systems are also known which
feature re-programmability of data via a radio link. However, such systems
operate at programming rates which are unacceptably slow in AFC systems.
High speed transfer of data from the reader to the non-volatile memory in
the transponder is essential in AFC systems in order to minimise the
duration of a "transaction "-- a "transaction" in this context is defined
as the sequence of identifying the transponder, reading some information
from an area of the transponder's data memory and then writing some new
information back to the transponder for storage in its memory--, which in
turn relates to the ability of the AFC system to achieve the desired high
throughput of passengers each carrying a transponder for fare payment.
For example, GB 2163324 A (Electromatic) describes a field-powered
transponder which may have a re-programmable memory. Information for
storage therein is presented sequentially to the transponder by a reader
using an elementary communications protocol, and the transponder requires
time to commit each received data element to its non-volatile memory
before it can accept a further data element for storage therein. In this
example prior art system, the non-volatile memory is fabricated using
electrically erasable and programmable read-only memory (EEPROM or E.sup.2
PROM) which is characterised by requiring approximately 10 ms to perform
an erase/write cycle, which is the mechanism by which new data is written
into the non-volatile storage array. This sequential process is
time-consuming, and since in a typical APC transaction numerous packets of
date are required to be written, this results in an unacceptably long
transaction time.
SUMMARY OF THE INVENTION
The present invention addresses all of the above mentioned deficiencies
observed in prior art systems, and further exhibits some additional
inventive steps which in combination serve to provide an ingenious and
advanced passive transponder which is particularly well suited to use in
AFC systems.
Simultaneous identification of numerous transponders by one reader is
achieved using the same techniques as those disclosed in co-pending
application PCT/CA91/00147. To summarize, shortly after experiencing an
excitation signal from a reader, the transponder sends an identification
message. Without further action from the reader, the transponder sends
further identification messages which are separated in time by a randomly
varying interval, and that interval is an integer multiple of the time
taken to transmit a single identification message. The interval between
successive identification messages from a particular transponder is
derived from a pseudo-random sequence generator, and the generators in
different transponders are not synchronised. Hence, the various
transponders submit respective identification messages in different time
slots. An enhancement to this prior art technique is described in detail
in the body of this specification, and forms an aspect of the present
invention.
"Selective programming" is a term which is used to describe the mechanism
by which a reader can write data to a nominated transponder privately and
without interference from other transponders or confusion over the
intended destination (ie. identity of the recipient transponder). This is
achieved in the system of the present invention by giving every
transponder a unique serial number which is used in defining the intended
recipient of a particular data message packet transmitted by a reader. In
this way a reader can be in control of numerous transponders
simultaneously, each of whose identities is known to the reader through
the simultaneous identification mechanism herein described. The reader can
thus transmit a packet of data or instruction to a particular transponder
by including the serial number of the destination transponder as an
essential element of the communications protocol employed between the
reader and transponders.
The speed of transfer of data from a reader to the non-volatile memory of a
transponder is maximised in the system of the present invention by
presenting a packet of data for storage rather than a number of elements
(bits or bytes, for example) sequentially, which in turn acts to minimise
the time needed to perform a transaction as previously defined herein.
Prior art systems such as that disclosed in GB 2163324 A (Electromatic),
suggest the possibility of storing information in a programmable memory
(an E.sup.2 PROM, for example) which may be achieved by a contact or
non-contact programming means, but no consideration is given to the need
to store a substantial amount of information quickly. Rather, the
programming process as described in this prior art system is a tedious
bit-by-bit process. In order to address this deficiency of prior art
systems, the transponder in the system of the present invention arranges a
serially received message packet from a reader so as to present it in
parallel to the specially structured non-volatile memory array for
simultaneous storage therein. Hence, the transponder of the present
invention commits a multitude of bytes (1 byte equals 8 bits) to the
non-volatile memory in the same time that a prior art "bit-by-bit"
sequential system presents at most just one byte, resulting in a dramatic
reduction over the prior art in the time taken to perform a transaction.
Additional aspects of the present invention which illustrate further
inventive steps over prior art systems are described in the Description of
a Preferred Embodiment, below, and may be summarised thus:
According to the present invention, the communications protocol which
supports bidirectional transfer of messages between transponders and
readers has been carefully designed to minimise data communications time
and thus reduce the critical element of transaction time for the system.
The transponder of the present invention is equipped with circuitry that
allows it to make an autonomous assessment of the amount of energy
available from the excitation field generated by a reader, which, in
field-powered transponder, is the transponder's only source of power for
operation, this being power that it must also use to perform storage of
data in the non-volatile memory. At various times during its operation,
the transponder makes an assessment of the available energy extracted from
the excitation field and returns a status flag in a message to the reader
to indicate whether there is sufficient energy to perform a programming
operation. Prior art systems such as that described in GB 2163324 A
(Electromatic) teach the use of an energy level sensing circuit to
determine when there is sufficient energy available from the excitation
field for correct operation in a primary identification mode. However,
there is no suggestion in the prior art of important energy assessment
measurement performed by the present invention, which is conducted with
the specific purpose of determining whether it is viable to commence a
programming operation.
In combination with the energy assessment technique, a further
characteristic of the present invention is the provision of means for
determining, if a programming operation is commenced, whether or not the
operation can be completed if the power source is suddenly removed
immediately after the transponder commences a programming cycle. This
technique is referred to herein as "programming outcome prediction", and
overcomes the critical problem suffered by prior art systems of partial
data storage caused by loss of power during a programming cycle.
In a preferred embodiment of the present invention, a reader sends
information to transponders by applying a modulating signal to the carrier
signal which creates an amplitude shift keyed (ASK) carrier envelops. This
simple technique is used to permit the design of an elementary amplitude
demodulater circuit in the transponder. But this simplicity has an
accompanying disadvantage in that the dynamic range of a simple
demodulater circuit can act to restrict the range of satisfactory
operating distance between the reader transmission antenna and the
transponder. Hence, according to an additional novel aspect of the present
invention, an automatic gain control (AGC) circuit is provided which acts
to progressively reduce the quality factor (Q) of the field detector
circuit thereby limiting the amount of carrier signal delivered to the
rectifier circuit within the transponder's control circuit. This in turn
increases the dynamic range of the demodulator circuit and hence the range
of distance over which the transponder can operate satisfactorily.
A further aspect of the present invention is the use of a technique termed
herein as "transmission hold-off". It will be shown in the following
description that the transponder is inherently capable of detecting the
presence of other transponders in an excitation field, and can make use of
this information to defer starting a message transmission when it first
experiences an excitation signal generated by a reader. This transmission
hold-off technique acts to further improve the simultaneous identification
performance of the system.
A yet further novel aspect of the present invention is the logical
separation between the functions of memory elements associated with
identification (ie the serial number of the transponder) and the
read/write data memory, which is only ever accessed when there is a need
to read or write data from or to the memory array. This has the distinct
advantage that the control circuit in the transponder can be designed to
minimise dynamic current consumption during the identification cycle,
thereby increasing the maximum possible range over which the transponder
may be identified.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the present invention is described herein below
with reference to the accompanying figures.
FIGS. 1a, 1b and 1c show a diagrammatic representation of a control circuit
for the transponder.
FIG. 2 illustrates how the control circuit shown in FIG. 1 may be
incorporated in a field-powered transponder.
FIG. 3 illustrates how the control circuit shown in FIG. 1 may be
incorporated in a self-powered transponder.
FIG. 4 shows a portion of the data memory and associated logic that
provides parallel storage of several data bytes in order to minimise
transaction time.
FIG. 5 shows a portion of the control circuit shown in FIGS. 1A-1C relating
to automatic gain control (AGC).
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring first to FIGS. 1-1C, a substantially sinusoidal excitation signal
from a reader (not shown) is detected by a tuned circuit (FIGS. 2 and 3)
external to the integrated circuit (IC) 1 at input terminals COIL1 and
COIL2. The construction and operation of such readers is well known in the
art, and no further detailed description thereof is provided herein. The
detected signal is passed to a bridge rectifier circuit 2 which acts to
rectify and regulate the detected AC signal. Energy level detector circuit
3 serves to generate a power-on reset and other signals which indicate to
control logic 9 whether there is sufficient energy for various operations.
The operation of this detector circuit is described in detail herein below
with reference to the programming operations associated with data memory
array 15.
Clock recovery circuit 17 extracts the clock signal from the excitation
signal, which is then used to derive timing signals for the remainder of
control circuit 1, through timing generator-block 8. Demodulation of
modulations in the excitation carrier signal are performed by demodulator
and discriminator 4, which then presents the demodulated information to
control logic 9 for processing. Security logic 5 acts to prevent
unauthorized access to the transponder's data memory 15.
All messages transmitted to the transponder by a reader and all messages
sent by a transponder to a reader include an error check field. CRC
(Cyclic Redundancy Check) accumulator 6 serves to assess the integrity of
received messages and dynamically compose the error check field which is
appended to transmitted messages.
Control logic 9 is combinational logic which functions so as to govern the
operation of the transponder in accordance with pre-defined operating
sequences, as discussed in greater detail below. Delay generator 7 is used
by control logic 9 to manager he simultaneous identification feature of
the present invention.
Information received by a transponder contains an instruction field which
is processed by instruction decoder 11 to determine what action is
required by the transponder in response to each received message. In
response to a received message, the transponder may generate a reply
message which also contains a message type field and this is generated by
a message type code generator 12 under control of control logic 9.
Data contained in a message received by the transponder is temporarily held
in scratchpad memory 13 before it may be written to data memory array 15
via address decoder and latch 14. Data memory array 15 may, for example,
be an array of CMOS E.sup.2 PROM cells, which requires a relatively higher
voltage for writing than reading. This voltage is produced by charge pump
16. Scratchpad memory 13 also serves to queue data fetched from data
memory array 15 via address decoder and latch 14 during a data fetch
cycle.
During the identification cycle which is entered shortly after the
transponder experiences an excitation signal from the reader, control
logic 9 accesses a special fixed code area 10 which holds the
transponder's serial number and other key information, without any need
for control logic 9 to access data memory array 15, thus saving dynamic
power.
Data and other information to be transmitted by the transponder is routed
via control logic 9 to modulation switch 18, which functions to vary the
current in the external tuned circuit (not shown) and thereby convey
information and data to the reader.
FIG. 2 shows a simple representation of an embodiment of the invention
wherein control circuit 1 is employed in a field-powered transponder. In
this configuration, the only power for operation is derived from the
excitation field generated by the reader (not shown). Detector coil 19 and
capacitor 20 form a parallel tuned circuit whose resonant frequency is
equal to the frequency of the excitation signal generated by the reader.
Although tuning coil 19 and capacitor 20 are shown as discrete components
and not as part of the IC 1, it is well known that such components can be
fabricated on-chip, but this does not in any way affect the operation of
the present invention.
The substantially sinusoidal signals transmitted by the reader and detected
by the tuned circuit comprising detector coil 19 and capacitor 20 deliver
a substantially sinusoidal signal to IC 1 at terminals COIL1 and COIL2. In
this field-powered mode of operation, the transponder circuit comprising
coil 19, capacitor 20 and control circuit 1 operates in the manner
described in greater detail below, without need for any additional
components or power source.
FIG. 3 shows a self-powered transponder, comprising detector coil 19 and
tuning capacitor 20, which detects and delivers signals generated by a
reader (not shown) to control circuit 1. The control circuit 1 is powered
by primary cell 21, which may for example be a lithium manganese-dioxide
chemistry cell capable of delivering a nominal terminal voltage of 3V.
Transmitter 22 is used by control circuit 1 to send response signals to the
reader via antenna 23. Transmitter circuit 22 is controlled by control
circuit 1 via two signals generated at output pins labelled TxON and MOD,
which prompt the transmitter circuit to enter a power-up or power-down
condition and provide a modulating signal for carrier modulation. It
should be noted that the transmitter in this configuration of control
circuit 1 is powered from cell 21 under control of IC 1 so that the power
required by the transmitter is drawn from cell 21 rather than from the
excitation field generated by the reader, as in the embodiment of FIG. 2.
The design of the transmitter 22 is well known and conventional, and is
capable of providing an identification system in accordance with
application demands, without affecting the principles of operation of the
present invention.
FIG. 4 shows a portion of data memory 15 and control logic 9 of control
circuit 1, which provides parallel storage of a large number of data
bytes. In this preferred embodiment of the present invention, data memory
15 has an architecture of 16 byte wide "pages" with a number "n" of such
pages being stored in the data memory array. Data is received by the
transponder bit-serially from the reader and is shifted into the
scratchpad memory 13. Before data writing to data memory array 15
commences, 16 bytes of data are accumulated in scratchpad memory 13,
assessed for errors by control logic 9 and CRC accumulator 6 (shown in
FIGS. 1A-1C), and If the data is error free the data is then presented to
data memory array 15 via address decoder and latch 14.
The time taken to store the accumulated 16 bytes of data in data memory
array 15 is equal to the time that would be required to store just one
byte in a prior art sequential storage system, so in practice the method
disclosed herein produces a storage time per byte that is 1/16th of the
time required to store 1 byte in a prior art sequential system.
The communications protocol between reader and transponder of the present
invention accommodates block transfer of data so that once a 16 byte page
of data has been sent to a transponder and determined to be error free
following it s receipt, there is only one acknowledgement required for
this block of 16 bytes, in contrast to prior art sequential systems which
typically have an acknowledgment from transponder to reader following
receipt of each individual data byte in a data packet.
FIG. 5 shows a portion of control circuit 1 (shown in full in FIGS. 1A-1C)
for implementing the AGC function of the present invention for limiting
the amount of signal extracted from the field generated by the reader and
delivered to remainder of control circuit 1 for operation. AGC element 24
is essentially a voltage controlled resistance such as a field effect
transistor (FET) which acts to progressively decrease the loaded Q of
detector circuit coil 19 and capacitor 20.
Theory of Operation
The communications protocol of the present invention has been designed to
accommodate robust exchanges of instructions and information
bidirectionally between readers and transponders, and particular attention
has been paid to the transaction process. To recap, a "transaction" is a
sequence of message exchanges between the reader and a nominated
transponder, followed by the reading of a portion of data from the
transponder's data memory array 15, writing of some fresh information into
the data memory array and obtaining a confirmation from the transponder
that the prescribed data storage action has been completed. The novel
method employed by the present invention follows three simple rules:
1. The reader sends a packet of data to the transponder for storage,
including an error check field so that the transponder may make a
self-assessment of the integrity of the received data packet using CRC
accumulator and comparator 6 in FIGS. 1A-1C under control of control logic
9, prior to presenting this data to the memory array 15 for storage
therein.
2. If the received message is error free, then no immediate response is
given to the reader by the transponder, thereby minimizing the transaction
time. Instead, data is transferred immediately to data memory array 15,
and when storage is determined to be complete by control logic 9 in FIGS.
1A-1C, a confirmatory acknowledgement message is sent to the reader for
reporting completion of the prescribed programming operation and the new
contents of the destination page is fetched from data memory array 15 and
transmitted as an element of the confirmatory message.
3. If however the received data message contains a detectable error, then
the transponder immediately transmits a special message to the originating
reader to indicate that a corrupted massage has been received, thereby
implicitly requesting the reader to re-transmit the message packet.
In this way, the overall transaction time is minimised for an error free
exchange, since time is not spent explicitly acknowledging receipt of an
error free message. Rather, the transponder performs the prescribed data
storage task and then reports completion of this operation, and a message
which is determined to contain errors is treated as an exception.
The aspect of the present invention described above as "selective
programming", is an important aspect in that it allows the transponder
system of the present invention to meet the demands of application in AFC
systems, or in other situations where more than one transponder may come
under control of a reader at a particular time and it is desired that the
reader can determine which particular transponder should receive a certain
transmitted message packet. It has been stated herein that every
transponder is given a unique serial number (stored in fixed code area
10), at the time manufacturing the control circuit 1. For reasons of
security and other operational advantages described herein, this serial
number is written to IC 1 using laser fusing techniques. It is this serial
number which is used by the reader to indicate to any transponders under
its control which transponder a particular message is intended for.
Furthermore, control circuit 1 is provided with special security logic
circuitry 5 (FIGS. 1A-1C), which encrypts and decrypts the serial number
portion of any message transmitted or received by a transponder. By a
proprietary mechanism performed by security logic 5, when a transponder
sends an identification or other message its unique serial number may be
transmitted in an encrypted format. A reader receiving such a message must
have knowledge of the encryption algorithm implemented by security logic 5
in order to be able to deduce and include the unique identification and
memory access code particular to the originating transponder. Otherwise,
the message packet is rejected and read or write access to the transponder
data memory array 15 is denied. In this way, dual functions are provided
of individually addressing a particular transponder for data reading and
programming and providing an encryption mechanism which results in a
write-access code that is unique to each transponder, which further acts
to advantageously minimise communication message lengths and hence reduce
transaction times.
It has been noted that for a field-powered transponder as shown in FIG. 2,
control circuit 1 is able to make an assessment of the amount of energy
available from the excitation field produced by a reader. This is
determined by energy level detector 3 which provides signals to control
logic 9 which can thereby make periodic assessment of the amount of energy
available for different internal operations. For example, storing data in
data memory array 15 when this is fabricated using CMOS E.sup.2 PROM cells
requires significantly greater power than when reading data therefrom.
Therefore, when control circuit 1 is requested to perform a data
programming operation following receipt of an instruction and associated
data packet from a reader, it makes an assessment of whether this
operation can be performed given the amount of energy available from the
excitation field. This is achieved by energy level detector 3 loading the
supply voltage delivered to the majority of control circuit 1 by regulator
2 with a dynamic load representative of the load presented during actual
writing to data memory array 15. The time taken to store information in
memory array 15 is known, as is the power required to perform the data
storage operation (ie. charge tunnelling in the case of CMOS E.sup.2
PROM). Hence, energy level detector circuit 3 can make a determination of
the amount of energy required to conduct the data storage process.
If there is sufficient energy available, then control logic 9 manages the
data writing operation in a predetermined manner. If there is insufficient
energy control circuit 1 sends a message to the originating reader using
the communications protocol described herein.
Furthermore, the aforementioned energy assessment technique provides the
transponder with the facility to predict the successful outcome of a data
writing operation even if the transponder's power source (the excitation
signal generated by the reader) is suddenly removed immediately after the
time energy assessment action is performed, since the assessment process
is performed on the basis of energy stored in the transponder's energy
storage device (not shown), which may, for example, be a capacitor and
which may be an integral part of rectifier and regulator circuit 2 or
connected to control circuit 1 via a signal input pin or terminal.
In the preferred application of the present invention in AFC systems, the
"outcome prediction" capability is a particularly powerful facility since
a passenger carrying the transponder and using it as an electronic ticket
may pass the transponder through the excitation field generated by the
reader so quickly that the reader has insufficient time to complete a
transaction in situations where multiple pages of data are to be written
to transponder. In the absence of the technique described herein, it would
be possible that a page of data may not be properly stored if the
transponder is suddenly removed from the excitation field during a data
writing operation, resulting in consequential premature volatility of the
data. The technique of outcome prediction described herein eliminates this
potential problem and provides a system which features reliable data
storage under such extreme conditions.
In practical installations of a transponder identification system, the
distance between transponders and the reader's antenna for generating an
electromagnetic excitation signal, is not fixed. Thus, the level of signal
detected and extracted by a transponder will vary greatly as the
transponder's position and orientation with respect to the driven antenna
changes. AGC circuit 24 in FIG. 5 has been introduced in the present
invention to give the transponder the greatest possible dynamic operating
range.
Although not shown in detail, the AGC circuit 24 is preferably essentially
a voltage controlled variable resistance element such as a field effect
transistor (FET) which is designed to act so that when the level of signal
detected by the transponder is below a predetermined threshold, the
resistance approaches infinity and therefore presents negligible load to
detector circuit coil 19 and tuning capacitor 20, resulting in the highest
possible loaded Q for the detector circuit and hence the greatest
efficiency. When the level of the signal is above this predetermined
threshold, a variable control signal is applied to AGC element 24 which
causes its resistance to progressively decrease, thereby presenting an
increasing load to the tuned detector circuit which in turn serves to
reduce its loaded Q and hence its efficiency as a detector.
This has the desirable effect of limiting the amount of signal available to
the transponder irrespective of how close it is to the antenna of the
reader which generates the electromagnetic field that excites and powers
the transponder. This, in turn, means that the dynamic range of the
transponder, and in particular its clock extraction circuit 17, is
significantly increased in comparison with a transponder which does not
incorporate such an AGC circuit.
In the field-powered application of control circuit 1 shown in FIG. 2, the
control circuit 1 is described as functioning not only to receive the
power and clocking signal via detector coil 19 and capacitor 20, but also
to send signals from the transponder to the reader. However, the
transponder circuit operates equally efficiently as a detector of signals
originating from other-transponders which may be in the vicinity of the
reader. Control circuit 1 incorporates circuitry for recognizing the
modulation format associated with a transponder's transmission to the
reader, and discriminator 4 and control logic 9 are arranged so as to be
sensitive to such modulation patterns. In this way, a particular
transponder may detect the near presence of other transponders when those
other transponders are transmitting to the reader.
When a transponder first enters the excitation field generated by a reader,
it initially determines whether there are any modulation patterns present
in that field consistent with those which would be observed when a
transponder is transmitting to the reader. Control circuit 1 is arranged
to operate such that if it detects patterns which are determined to have
originated from another transponder immediately after entering the
excitation field, it "holds off" transmitting its identification message
until the detected transmission is finished. This serves to enhance the
simultaneous identification performance of the system and improve
communications between reader and the transponders, and between the
transponders and reader.
As discussed above, each transponder is provided with a unique serial
number at the time of fabricating control circuit 1. This number is etched
into the chip in special area 10 (FIGS. 1A-1C) using laser fusing
techniques, and so presents a fixed code. This permanently stored
identifier data is conceptually and logically separated from read/write
data memory 15. Serial number area 10 is accessed by control logic 9 in a
predetermined manner in order to complete the message transmission and
reception operations defined in the communications protocol between
transponder and reader. Control logic 9 is able to access serial number
code area 10 without seeking access to data memory array 15 via functional
blocks 13 and 14, which means that power consuming circuits 13 and 14
which are associated with accessing read/write data memory array 15, need
not be enabled in order for the device to simply transmit an
"identification only" message, comprising serial number, error check and
framing fields. This results in a further minimisation of overall dynamic
power consumption and hence increased range in the identification mode.
By reference to the preceding description of a preferred embodiment of the
present invention and to the accompanying drawings, it would be a
straightforward exercise for one skilled in the art of silicon integrated
circuit design to devise the detailed logic and analogue circuit elements
required for the various functional blocks shown in FIGS. 1A-1C to
implement the functionality herein described and claimed.
The preferred embodiment herein described and depicted is given byway of
practical example only, and it will be readily appreciated to one skilled
in the art of designing transponder systems that there can be many
possible variations in implementation of such a transponder which permit
fabrication of devices that retain close adherence to the operating
principles described and claimed herein.
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